The present invention relates to a complex NPK fertiliser in the form of granules or prills, and to a method for its preparation and to the related use of the fertiliser thereby obtained. In particular, the present invention relates to a fertiliser containing as fertilising elements Nitrogen N, Phosphor P, Potassium K, meso elements like Calcium Ca, Magnesium Mg, Sulphur S, and micro elements like Iron Fe, Manganese Mn, Zinc Zn, Copper Cu, Boron B and Molybdenum Mo.
Complex NPK fertilisers containing meso and micro elements are known in the trade. These types of fertilisers are obtained by etching phosphatic rocks with sulphuric, nitric or phosphoric acid with the subsequent ammonification and addition of Potassium in the form of potassium sulphate or potassium chloride to obtain a suspension or pulp. The solution or pulp is then transformed into solid particles shaped as granules or prills, by means of granulating processes or through the prilling of the molten salts. Although these kinds of fertilisers contain in full or in part the nutrients necessary for crops, they are not free of disadvantages. One disadvantage is that of not being perfectly water soluble due to the presence of the residues deriving from the etching of the phosphatic rocks, i.e. if solubilised they leave an insoluble residue. The direct application of the solution of complex NPK fertilisers thereby obtained causes the blockage of distribution systems in fertilisation-irrigation, leaf fertilisation, and hydroponic fertilisation systems.
There are patents that relate to the production of water soluble complex NPK fertilisers that contain meso and micro elements.
All such methods provide for obtaining a solution of molten salts containing N, generally NH4 NO3, with the subsequent addition of solid salts containing P such as NH4H2PO4 or (NH4)2HPO4 and/or K such as K2SO4 and KCl. The limitation for these methods consists of the fact that the added phosphatic and/or potassic salts are in solid form and can give rise to a suspension, leading to problems of inconsistency in the molten solution in addition to high viscosity levels, which as is well known prevent the perfect homogenisation of the solution.
It is known that the production of highly soluble complex NPK fertilisers available on the market is carried out by mixing the various sources of N, P, K, perfectly soluble meso and micro elements, after grinding the components to obtain a uniform grain size. This common practice presents two disadvantages. The first disadvantage is that the product thereby obtained contains dusts due to the grinding, creating problems in any direct application. The second disadvantage is that of having to mix together components having different density with the consequent problems of component segregation during the preparation, transport, storage of the sacks and during the mechanical distribution on the soil.
Also commercially available are products which do not present segregation or dustiness problems, both in the form of granules, prills or crystals which are perfectly water soluble, but bring to the crops only two nutrients among N, P, and K and at times meso and micro elements. I.e., NP, KP, and NK fertilisers are available.
It is also well known that all fertilisers have the tendency to become compacted due to the irregularity of the particles, lack of homogeneity of the particles, and residual humidity.
One of the aims of the present invention is to provide a method which allows to obtain a water soluble complex NPK fertiliser in granules or prills, highly soluble in water, which may contain meso and micro elements and has perfectly predictable composition characteristics since each granule or prill derives from a solution with homogeneous composition.
Another aim of the proposed method is to provide a water soluble complex NPK fertiliser in granules or prills, which does not present segregation phenomena during the preparation, transport or storage of the sacks and in the course of mechanical distribution on the soil, and which contains no dust.
Yet another aim of the proposed method is to provide a water soluble complex NPK fertiliser in granules or prills which can be used in localised fertilisation-irrigation or in widespread fertilisation-irrigation, leaf fertilisation, hydroponic fertilisation, or usable also when the immediate availability of nutrients is required, but no specific application device is available and the product is distributed on the soil, with subsequent irrigation by traditional methods.
A further aim of the method proposed by the present invention is to provide a water soluble complex NPK fertiliser in granules or prills, having such composition and homogeneity characteristics as to provide it with greater resistance against compacting.